Astrecks

Imperial or Metric scale ?

I make no apologies for using a metric scale throughout these web pages in view of the fact that Ordnance Survey (OS) started to re-triangulate the whole of the UK on a metric scale back in 1936 and has nothing to do with interference from the EC (pah spit!). All UK 1:25 000 & 1:50 000 maps are now metric and I recommend you base all your routes in metric measurement to save time and possible errors in trying to convert to imperial measurement.

Ordnance Survey – a brief history.

Ordnance Survey (O.S.) was founded by an act of parliament in 1791 to produce high quality maps for military purposes. In 1799 the threat of a French invasion accelerated the need to survey the south coast, and so in 1801, the first O.S. map of Kent at one inch to the mile was published. Maps at many different scales were produced and by 1842 the need to standardise map scales started a "battle of the scales". This wasn’t resolved until 1863 when one inch to one mile maps was established as a general purpose topographical maps, right up to the enormous ten feet to the mile for built up areas. By 1914 the one inch to the mile maps became very popular with the public, and O.S. saw the potential for massive sales to the public of the one inch series. The Great War put a hold on this project until the end of the war. 1936 saw the start of re-triangulation of Great Britain on a metric grid. The construction of over 6,500 of the now familiar "trig points" was to take until 1962 to complete. 1969 saw the opening of the O.S. new purpose built Headquarters in Maybush, Southampton and in 1983 O.S. became a civilian organisation. O.S. have always been at the forefront of the technology of the day and as such, a ready supply of periodically revised maps have always been available. Today the National Topographic Database, holds over 200 million features of the British landscape, a seamless map of the whole of Great Britain that is updated on a daily basis by O.S. field staff using hand held computers to survey and digitise information. For more on the history of O.S. follow this link: http://www.ordnancesurvey.co.uk/oswebsite/aboutus/history.html

The compass explained.

Probably the best type of compass to use in conjunction with a map looks like the diagram below and these are made by several manufacturers and range from around £12 to £60. They comprise of a clear acrylic base-plate usually with scale markings, the bezel which is normally liquid filled to dampen the motion of the magnetic needle. The bezel is free rotating and the outer ring is marked with the degree scale usually in two degree intervals ( the more expensive prismatic models can be read to half a degree ). The base of the bezel has orienting lines to enable you to line them up with the gridlines on your map.

 

When using an Ordnance Survey (O.S.) map in conjunction with a compass there are 3 north’s to take into account, these are:-

TRUE NORTH.

True north points in the direction of the North Pole, and unless you are planning to go to the North Pole it is not necessary to be concerned with this on a local basis.

GRID NORTH.

The vertical gridlines on an O.S. map are a consequence of trying to represent the spherical shape of the earth on a flat sheet of paper. In order for these gridlines to meet at the poles, in the case of the northern hemisphere, the lines at the bottom of the map would be further apart than those at the top. Adjoining maps would be difficult to produce. For this reason, the gridlines are parallel and therefore the land as represented on the map is actually a distorted view, which in the case of larger scale maps is negligible.

MAGNETIC NORTH.

Magnetic north changes over time due to the earth’s molten core which in basic terms "sloshes" about due to forces exerted upon it by the climatic conditions, the gravitational effects of the moon and magnetic influences of solar flares, local variations have to be taken into account, for example the Cuillin Hills in the Isle of Skye have a high iron ore content and due to its volcanic formation can have an effect on magnetic variation. Magnetic variation is predicted by several international organisations, International Geomagnetic Reference Field, being one such organisation.

When using a compass we have to know how to calculate the difference between GRID NORTH and MAGNETIC NORTH, and when to add or subtract this variation. This depends on two things:-

Whether magnetic north is west or east of grid north & whether you are transferring bearings from map to terrain or terrain to map.

1. When using a map, the saying "West is best, east is least" meaning if magnetic north is west of grid north then ADD the variation and if magnetic north is east of grid north then SUBTRACT.

2. When transferring a bearing taken on a landmark the reverse of the above is true.

An O.S. map tells you what the difference will be at a given year and in what direction magnetic north is moving, e.g. :-

"Magnetic north is estimated at five and a half degrees west of grid north for 1993 decreasing by about a half degree in three years" (refer to No. 1. Above)

This means that in 1996 magnetic north would be five degrees west of grid north and in 1999 four and a half degrees west of grid north and so on. These half degrees are not critical unless you were travelling great distances on a bearing only. As the markings on most compass bezels are at two degree intervals it is easier to round down to the nearest whole degree.

If you are going to use an old map, say more than twenty years old then it would be better to calculate the magnetic variation yourself, as magnetic variation doesn’t change at a constant rate and its movement can speed up or slow down, the original estimated movement maybe well out. This can be done if you know your exact position and can identify a landmark in the distance which you can locate on your map. Take a bearing on the object and note the reading. Using your compass as a protractor, do the same on your map, the difference between the two readings is the magnetic variation. The further away your chosen landmark is, the more accurate the variation figure will be.

PLOTTING A COURSE USING A MAP.

Place your compass on the map with the long edge pointing in the direction of your intended course, (a) your present location and (b) your intended destination. Turn the compass bezel until the bezel lines line up with the north/south gridlines on your map, make sure the northern bezel lines (usually coloured red are oriented to the north of your map, even if your intended line of travel is south) then read off the bearing, adjust for magnetic variation. Your compass is now set for you to travel to your destination. Hold the compass flat in your hand and rotate yourself until the red magnetic arrow settles in line with the north bezel arrow, the direction of travel arrow will now be pointing funnily enough in the direction of travel. Pick a landmark or object in line with your direction of travel and walk to it. Be sure the landmark or object is no further away than your intended destination, otherwise you may walk right past it!

 

 

AIMING OFF.

Suppose you need to cross a river and you needed to find a suitable crossing point such as a bridge or stepping stones. You find a suitable place marked on the map, if you plotted the exact bearing to the crossing as indicated by (A) then depending on the nature of the terrain on which you are walking, you could possibly deviate off course enough to miss the crossing (C). How ever if you deliberately aim off course (B) when you arrive at the river you can reasonably expect to have to turn left and follow the river until you come to the crossing.

 

SQUARING.

Squaring is used if it is not possible to walk directly along a bearing because an obstacle prevents this. To do this walk as near the object as possible then turn 90 degrees left or right, then pace count in that direction enough to bypass the object turn 90 degrees again and continue on bearing until you pass the obstacle, again turn 90 degrees and pace back the same number of paces as for your first turn, again turn 90 degrees then continue on your bearing. See example below.

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The whole of Britain is mapped in 100 x 100 kilometre sections (100 km²)  Each sections is referenced with two letters, e.g.  SH  SJ and SK.  Each of these sections are further divided into smaller 10 x 10 kilometre sections (10 km²). Each of these smaller sections are given a number e.g. the bottom left section of  the SK sheets would be numbered SK 00  and the top right section of the  SK sheet would be SK 99. As you can see below, I have been too lazy to fill in all the squares with references, but I'm sure you get the idea!

 

 

 

When quoting grid references the figures for the Eastings come before the Northings. A way to remember this is that Eastings are alphabetically before Northings or the expression "Along the corridor and up the stairs".

 

In the example below we are using the SK 100 x 100 km sheet.

 

 

 

We will then narrow this down to the 10 x 10 km square SK 7 3,  as shown in the flyout.

The next diagram  is represented by the yellow squares in the 10 x10 km square above. We will now narrow it down further to a 1 x1 km square (the red square).

This will give us a grid reference of SK 73 33. We want to give a more defined grid reference for the red triangle in the diagram below by adding a third Eastings reference and a third Northings reference.

Now that we have narrowed it down to a 1 x 1 km square, we can use the handy grid referencing scales on the compass body to get the grid reference down to 100 x 100 metres. 

 

  

 

 

Note on Silva Compasses there are scales for:-

1:25 000.

1:50 000.

1:63 360 (1 inch to 1 mile).

 

The final grid reference would be  SK 735 336.

 

This is about as defined as one can get without GPS technology or a very big magnifying glass!. 

 

Ordnance Survey Explorer maps cover an area of approx. 30km x 45km and different 100 x 100 km sections can come together on one map. Where they do join, the sheet section letters are displayed. These occur where the Eastings and Northings are at 00. It is most important that this is recognised when giving grid references. 

 

 

 

 

Disclaimer.

 

I would like to point out that I am not a professionally qualified mountaineer and cannot be held responsible for anyone who ventures out onto the mountains and remote areas of the UK based on the information contained within this web-site.

Only the fool hardy would venture out on the hills without the correct clothing & equipment and the ability to use them.

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